The purpose of this study is to determine whether significant drug interactions occur in opioid-dependent patients without HIV disease who are administered treatment for opioid dependence (methadone, LAAM or buprenorphine) and are administered the protease inhibitor nelfinavir (Viracept). The goal of these types of studies is to enhance the treatment of patients with HIV infection and opioid dependence, improve outcome, and decrease risk of transmission of the virus through a better understanding of clinically significant medication interactions between opioids and antiretroviral therapies.